1. 1
DNA
The illustration is a ‘model’ of the double helix forming part of a DNA molecule
(Slide 14)

2. DNA stands for deoxyribonucleic acid2
DNA stands for deoxyribonucleic acid
This chemical substance is present in the nucleus
of all cells in all living organisms
DNA controls all the chemical changes which
take place in cells
The kind of cell which is formed, (muscle, blood,
nerve, etc.) is controlled by DNA
The kind of organism which is produced (flower,
giraffe, herring, human, etc.) is controlled by DNA

3. DNA is a very large molecule made up of a long chain of sub-units
DNA molecule 3
DNA is a very large molecule made up of a long
chain of sub-units
The sub-units are called nucleotides
Each nucleotide is made up of :
1. a sugar called deoxyribose
2. a phosphate group -PO4 and
3. an organic base

4. Ribose is a sugar, like glucose, but with only five
Ribose & deoxyribose 4
Ribose is a sugar, like glucose, but with only five
carbon atoms in its molecule
Deoxyribose is almost the same but lacks one
oxygen atom
Both molecules may be represented by the symbol

5. The most common organic bases are
The bases 5
The most common organic bases are
Adenine
(A)
Thymine
(T)
Cytosine
(C)
Guanine
(G)

6. combine to form a nucleotide
Nucleotides 6
The deoxyribose,
the phosphate
and one of the bases
combine to form a nucleotide
adenine
deoxyribose
PO4
It is the phosphate group which gives DNA its acidic properties

7. Joined nucleotides 7
PO4
sugar-phosphate
backbone
+ bases
A molecule of DNA is formed by millions of nucleotides joined together in a long chain

8. In fact, the DNA usually consists of a double strand of nucleotides8
In fact, the DNA usually consists of a double
strand of nucleotides
The sugar-phosphate chains are on the outside
and the strands are held together by chemical
bonds between the bases

9. 2-stranded DNA 9
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4

10. The bases always pair up in the same way
Bonding 1 10
The bases always pair up in the same way
Adenine forms a bond with Thymine
Adenine
Thymine
and Cytosine bonds with Guanine
Cytosine
Guanine

11. Chargaff’s Rule
In 1950, Erwin Chargaff analyzed the base composition of DNA composition in a number of organisms.
He reported that DNA composition varies from one species to another. Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein.

12. Chargaff’s rule: C = G and T = A
Pyrimidines =
Cytosine and Thymine
Purines =
Guanine and Adenine
(There are two kinds of nitrogen-containing bases - purines and pyrimidines. Purines consist of a six-membered and a five-membered nitrogen-containing ring, fused together. Pyrimidines have only a six-membered nitrogen-containing ring.)

13. Bonding 2 11
PO4
adenine
cytosine
PO4
thymine
PO4
guanine
PO4
PO4

14. Pairing up 12
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4

15. DNA Orientation
On the top rail, the strand is said to be oriented 5′ to 3′.
The strand on the bottom runs in the opposite direction and is oriented 3′ to 5′.

16. (Resembles a twisted ladder)13
The paired strands are coiled into a spiral called
A DOUBLE HELIX
(Resembles a twisted ladder)

17. 14
THE DOUBLE HELIX
bases
sugar-phosphate
chain

18. A DIY model of part of a DNA molecule15
A DIY model of part of a DNA molecule
This is a teaching model. The colours of the bases do not conform to the accepted
scheme today

19. Rosalind Franklin Found Helical pattern Backbone on outside
Antiparallel (run parallel to each other
but with opposite alignments)

20. Watson and Crick
Made model

21. Stop DNA Structure

22. DNA REPLICATION
DNA needs to replicate in order for a cell to undergo cellular division.
Before a cell divides, the DNA strands unwind and separate.

23. 17
PO4
PO4
The strands separate

24. The enzyme DNA helicase is responsible
replication 16
The enzyme DNA helicase is responsible
for unwinding and unzipping the double helix.

26. DNA polymerase (an enzyme) continues adding appropriate nucleotides to the chain by adding to the 3′ end of the new DNA strand. (Towards the replication fork- called “5’ to 3’ direction”)

27. One strand is called the leading strand and is elongated as the DNA unwinds so is said to be synthesized continuously. Towards replication fork.
The other strand of DNA, called the lagging strand, elongates away from the replication fork.
The lagging strand is synthesized discontinuously into small segments, called Okazaki fragments

28. Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication

29. 18
Each strand builds up its partner by adding the appropriate nucleotides
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
PO4
The nucleotides are present in the nucleoplasm. The nuclear equivalent of cytoplasm
PO4
PO4
PO4
PO4
PO4

30. DNA ligase links the two sections.

31. The result is that there are now two double-stranded DNA molecules in the nucleus
Now when the cell divides, each nucleus contains identical DNA

32. The end product will be two identical copies of DNA.
UNLESS…….there are mutations
Mutations are mistakes in DNA and occur 1 in 10,000 base pairs.

33. DNA REPAIR
Group of +50 enzymes that recognize, remove it, and repair it
Mutations can be harmful
Cancer
Mutations can be beneficial
Creates variation in a species which may be good

35. Stop point - replication

36. The sequence of bases in DNA forms the
Genetic code 1 19
The sequence of bases in DNA forms the
Genetic Code
A group of three bases (a triplet) controls
the production of a particular amino acid in
the cytoplasm of the cell
The different amino acids and the order in which they are joined up determines the sort of protein being produced

37. This is a small, imaginary protein molecule showing
Genetic code 2 20
This is a small, imaginary protein molecule showing
how a sequence of 5 different amino acids could determine the shape and identity of the molecule
Ser-Cyst-Val-Gly-Ser-Cyst Ala
Val
Val-Cyst-Ser-Ala-Ser-Cyst-Gly
Val- Cyst-Ala-Ala-Ser-Gly
Each amino acid (Serine, Cysteine, Valine, Glycine and
Alanine) is coded for by a particular triplet of bases

38. Valine Alanine For example Codes for Codes for 21 Cytosine Adenine
Coding 21
For example
Cytosine
Adenine
Valine
Codes for
Thymine
Cytosine (C)
Alanine
Codes for
Guanine (G)
Adenine (A)

39. This is known as the triplet code22
This is known as the triplet code
Each triplet codes for a specific amino acid
CGA - CAA - CCA - CCA - GCT - GGG - GAG - CCA -
Ala
Val
Gly
Gly
Arg
Pro
Leu
Gly
The amino acids are joined together in the correct
sequence to make part of a protein
Although the DNA in the nucleus specifies the amino acids and their sequence, it is in the cytoplasm that the protein build-up takes place. The DNA of the nucleus makes a single strand of messenger RNA (ribo-nucleic acid) which leaves the nucleus and builds up the protein in the cytoplasm. The RNA code is complementary, but not identical, to the nuclear DNA. Text books usually give the coding for the RNA but in this presentation it is for the DNA itself
Ala
Val
Gly
Gly
Arg
Pro
Leu
Gly

40. The proteins build the cell structures
DNA and enzymes 23
The proteins build the cell structures
They also make enzymes
The DNA controls which enzymes are made and
the enzymes determine what reactions take place
The structures and reactions in the cell determine
what sort of a cell it is and what its function is
So DNA exerts its control through the enzymes

41. A sequence of triplets in the DNA molecule may
Genes 24
A sequence of triplets in the DNA molecule may
code for a complete protein
Such a sequence forms a gene
There may be a thousand or more bases in
one gene

42. Question 1 Which of the following are components of nucleotides?
(a) deoxyribose
(b) amino acids
(c) phosphate
(d) enzymes
(e) organic bases

43. Question 2 Which of the following represent a correct
pairing of bases?
(a) adenine with thymine
(b) adenine with guanine
(c) thymine with adenine
(d) guanine with cytosine
(e) thymine with thymine

44. Question 3 DNA molecules are formed from (a) organic bases
(b) amino acids
(c) deoxyribose
(d) nucleotides

45. Question 4 Which of the following are organic bases? (a) Valine
(b) Guanine
(c) Thymine
(d) Serine

46. Question 5 Replication of DNA occurs (a) During cell division
(b) before cell division
(c) at any time

47. Question 6 A nucleotide triplet codes for (a) a protein
(b) an amino acid
(c) an enzyme
(d) an organic base

48. Answer
CORRECT

49. Answer
INCORRECT